1. Field of the Invention
The present invention relates in general to a field-effect transistor epitaxial structure. More particularly, the present invention relates to a novel gate metal used for Field Effect Transistors (FETs) and High Electron Mobility Transistors (HEMTs).
2. Description of the Related Art
Today, FETs and HEMTs fabricated for microwave and millimeter wave applications commonly use a three-layer combination of titanium (Ti)/platium (Pt)/gold (Au) as a gate metal because of the outstanding electrical and thermal stability of the combination. The effects of hydrogen on FETs or HEMTs with Ti/Pt/Au gates, however, has become a concern when using these devices in a hydrogen environment such as in hermetically sealed packaging.
Though the combination of a Ti/Pt/Au gate metal allows for a reliable, low resistance contact to the device, researchers have recognized Pt as responsible for the failure of hermetically sealed devices, see W. O. Camp, Jr., et al., "Hydrogen Effects On Reliability of GaAs MMICs," Proc. GaAs IC Symp., pg. 203, 1989. In a hydrogen environment, a catalytic reaction of hydrogen (H.sub.2) with Pt results in the formation of atomic hydrogen (H), resulting in the decreased performance of the device.
Researchers have also recognized the degradation of InP HEMT performance in a hydrogen environment, see P. C. Chao et al., "HEMT Degradation in Hydrogen Gas," IEEE Device Letters, Vol. 15, No. 5 (May 1994). In InP HEMTs with Ti/Pt/Au gates, the hydrogen reaction creates a shift of the gate built-in potential, resulting in a threshold voltage shift toward more positive or more negative depending on the construction of the device. The shift in threshold voltage increases or decreases the device drain current at a fixed gate bias and effects the performance of the device. Potential candidates for a hydrogen gate metal, such as tungsten, are not suitable because they require deposition techniques which are more complex and costly.
Therefore, there is a need for a gate metal structure which is insensitive to hydrogen's effects on FET and HEMT performance, economical, and compatible with existing structures and manufacturing techniques. The new gate metal must also be free from electromigration effects, reliable under operating conditions, and a good electrical conductor.